The present invention is related to an air-to-fuel ratio control apparatus improved to control the air-to-fuel ratio of a combustible mixture supplied to a combustion engine toward a lean limit in response to the combustion condition of the combustible mixture which is detected by the comparison of the pressures in the engine cylinder produced during the compression and power strokes.
It has been known in the art field related to engine controls that a combustion engine emitts, so long as it attains a complete combustion therein, less amount of the hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxide (NOx) as the air-to-fuel ratio of a combustible mixture becomes a leaner ratio. Keeping the combustible mixture at the leanest possible ratio is effective not only to reduce noxious exhaust emissions but also to enhance fuel economy.
To derive these advantages a closed loop lean limit engine roughness control has been suggested in the U.S. Pat. No. to Taplin et al 3,789,816 issued on Feb. 5, 1974. The engine roughness in rotation is detected and the combustible mixture is responsively feedback-controlled in this patent. The combustible mixture is kept controlled to the leaner ratio so long as the detected engine roughness is small enough. Since the engine roughness is detected in view of the changes in the rotation of an engine output shaft, it is difficult to discriminate whether the engine roughness is resultant from the combustible mixture feedback-controlled to the leaner ratio or from the transient acceleration or deceleration of the combustion engine.